Cardiac event sensing in an implantable medical device

ABSTRACT

An implantable medical device performs a method that includes detecting a cardiac event interval that is greater than a P-wave oversensing threshold interval. In response to detecting the cardiac event interval greater than the P-wave oversensing threshold interval, the device determines the amplitude of the sensed cardiac signal and withholds restarting a pacing interval in response to the amplitude satisfying P-wave oversensing criteria. A pacing pulse may be generated in response to the pacing interval expiring without sensing an intrinsic cardiac electrical event that is not detected as a P-wave oversensing event.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of pending U.S. patent applicationSer. No. 16/804,877, filed on Feb. 28, 2020 (published as U.S.Publication No. 2020/0197708), which is a Continuation of U.S. patentapplication Ser. No. 15/497,546, filed Apr. 26, 2017, granted as U.S.Pat. No. 10,576,288, the content of both of which is incorporated hereinby reference in its entirety.

TECHNICAL FIELD

The disclosure relates generally to implantable medical devices (IMDs)and methods for sensing cardiac electrical events from a cardiacelectrical signal and in particular to methods for detecting oversensingof atrial P-waves attendant to atrial depolarizations.

BACKGROUND

Medical devices, such as cardiac pacemakers and implantable cardioverterdefibrillators (ICDs), provide therapeutic electrical stimulation to aheart of a patient via electrodes carried by one or more medicalelectrical leads and/or electrodes on a housing of the medical device.The electrical stimulation may include cardiac pacing pulses and/orcardioversion/defibrillation (CV/DF) shocks.

The medical device may sense cardiac electrical events attendant to theintrinsic heart activity for detecting an abnormal intrinsic heartrhythm. Upon detection of an abnormal rhythm, such as bradycardia,tachycardia or fibrillation, an appropriate electrical stimulationtherapy may be delivered to restore or maintain a more normal rhythm ofthe heart. For example, an ICD may deliver pacing pulses to the heart ofthe patient upon detecting bradycardia or tachycardia or deliver CV/DFshocks to the heart upon detecting tachycardia or fibrillation.

The ICD may sense the cardiac electrical signals from a heart chamberand deliver electrical stimulation therapies to the heart chamber usingendocardial electrodes carried by transvenous medical electrical leads.In other cases, a non-transvenous lead may be coupled to the ICD, inwhich case the ICD may sense cardiac electrical signals and deliverelectrical stimulation therapy to the heart using extra-cardiovascularelectrodes. Cardiac event signals, such as atrial P-waves andventricular R-waves, sensed by electrodes positioned within the atrialor ventricular heart chamber from which the signal arises, generallyhave a high signal strength for reliably sensing the cardiac electricalevents. In other examples, a non-transvenous lead may be coupled to theICD, carrying electrodes positioned at extra-cardiovascular locations,in which case the cardiac event amplitudes may have a relatively loweror more variable signal strength and/or different relative amplitudesbetween R-waves, T-waves and P-waves in the cardiac electrical signal.Reliable sensing of cardiac events by a pacemaker of ICD is important indetermining when an electrical stimulation therapy is needed anddelivering the appropriate electrical stimulation therapy for treatingan abnormal heart rhythm.

SUMMARY

In general, the disclosure is directed to techniques for sensing cardiacevent signals from a cardiac electrical signal and in particular foridentifying P-wave oversensing. An IMD operating according to thetechniques disclosed herein is configured to sense cardiac events,identify oversensed P-wave events, and neglect sensed events identifiedas oversensed P-waves in inhibiting pacing pulses and controlling aventricular pacing interval and/or in determining a ventricular rate fordetecting ventricular tachyarrhythmias. The IMD may operate to detectP-wave oversensing (PWOS) by a comparative analysis of the maximum peakamplitudes of two different cardiac electrical signals received by theIMD.

In one example, the disclosure provides an IMD including a therapydelivery circuit configured to generate cardiac pacing pulses, a sensingcircuit configured to receive a cardiac signal from a patient's heartvia sensing electrodes and sense intrinsic cardiac electrical eventsfrom the cardiac signal, and a control circuit coupled to the sensingcircuit and the therapy delivery circuit. The control circuit isconfigured to start a pacing interval and detect an event interval thatis greater than a P-wave oversensing threshold interval. The eventinterval is determined as the time interval extending from an intrinsiccardiac electrical event sensed by the sensing circuit from the cardiacsignal to a most recent preceding cardiac event. The control circuitdetermines an amplitude of the cardiac signal in response to detectingthe event interval greater than the P-wave oversensing thresholdinterval, withholds restarting of the pacing interval in response to atleast the amplitude satisfying P-wave oversensing criteria, and controlsthe therapy delivery circuit to generate a pacing pulse in response tothe pacing interval expiring.

In another example, the disclosure provides a method including sensingintrinsic cardiac electrical events from a cardiac signal received by asensing circuit of an implantable medical device from a patient's heartvia sensing electrodes, starting a pacing interval by a control circuitof the implantable medical device and detecting an event interval thatis greater than a P-wave oversensing threshold interval. The eventinterval extends from an intrinsic cardiac electrical event sensed fromthe cardiac signal by the sensing circuit to a most recent precedingcardiac event. The method further includes determining an amplitude ofthe cardiac signal in response to detecting the event interval greaterthan the P-wave oversensing threshold interval and withholdingrestarting of the pacing interval in response to at least the amplitudesatisfying P-wave oversensing criteria. The method includes generating apacing pulse by a therapy delivery circuit of the implantable medicaldevice in response to the pacing interval expiring.

In another example, the disclosure provides a non-transitory,computer-readable storage medium storing a set of instructions which,when executed by a control circuit of an IMD, cause the IMD to senseintrinsic cardiac electrical events from a cardiac signal, start apacing interval, detect an event interval that is greater than a P-waveoversensing threshold interval where the event interval extends from anintrinsic cardiac electrical event sensed from the cardiac signal to amost recent preceding cardiac event. The IMD is further caused todetermine an amplitude of the cardiac signal in response to detectingthe event interval greater than the P-wave oversensing thresholdinterval, withhold restarting of the pacing interval in response to theamplitude satisfying P-wave oversensing criteria, and generate a pacingpulse in response to the pacing interval expiring.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the apparatus and methods described indetail within the accompanying drawings and description below. Furtherdetails of one or more examples are set forth in the accompanyingdrawings and the description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are conceptual diagrams of an extra-cardiovascular ICDsystem according to one example.

FIGS. 2A-2C are conceptual diagrams of a patient implanted with theextra-cardiovascular ICD system in a different implant configurationthan the arrangement shown in FIGS. 1A-1B.

FIG. 3 is a schematic diagram of the ICD of FIGS. 1A-2C according to oneexample

FIG. 4 is a diagram of circuitry included in the sensing circuit of theICD of FIG. 3 according to one example.

FIG. 5 is a diagram of two cardiac electrical signals provided by thesensing circuit of FIG. 4.

FIG. 6 is a flow chart of a method for detecting P-wave oversensing(PWOS) according to one example.

FIG. 7 is a timing diagram depicting an example of PWOS detection andpacing control that may be performed by the ICD of FIG. 3.

DETAILED DESCRIPTION

In general, this disclosure describes techniques for detecting P-waveoversensing by a cardiac medical device or system. A cardiac medicaldevice may be configured to sense R-waves attendant to ventriculardepolarizations for use in controlling ventricular pacing. A ventricularpacing interval may be started in response to sensing an intrinsicR-wave and if the pacing interval expires before another R-wave issensed, a pacing pulse is delivered. In some instances, atrial P-wavesattendant to atrial depolarizations may be oversensed as R-waves. Anoversensed P-wave may cause the cardiac medical device to inhibit aventricular pacing pulse when the pacing pulse is actually needed tomaintain the ventricular rate at a programmed lower pacing rate. Byidentifying a sensed cardiac event as an oversensed P-wave, P-wavesfalsely sensed as R-waves may be neglected in inhibiting pacing pulsesand controlling a ventricular pacing interval and/or in determining aventricular rate for detecting ventricular tachyarrhythmias.

In some examples, the cardiac medical device system may be anextra-cardiovascular ICD system. As used herein, the term“extra-cardiovascular” refers to a position outside the blood vessels,heart, and pericardium surrounding the heart of a patient. Implantableelectrodes carried by extra-cardiovascular leads may be positionedextra-thoracically (outside the ribcage and sternum) orintra-thoracically (beneath the ribcage or sternum) but generally not inintimate contact with myocardial tissue. The techniques disclosed hereinfor detecting P-wave oversensing may be applied to a cardiac electricalsignal acquired using extra-cardiovascular electrodes.

These P-wave oversensing techniques are described herein in conjunctionwith an ICD and implantable medical lead carrying extra-cardiovascularelectrodes, but aspects disclosed herein may be utilized in conjunctionwith other cardiac medical devices or systems. For example, thetechniques for detecting P-wave oversensing as described in conjunctionwith the accompanying drawings may be implemented in any implantable orexternal medical device enabled for sensing intrinsic cardiac electricalevents from cardiac signals received from a patient's heart via sensingelectrodes, including implantable pacemakers, ICDs or cardiac monitorscoupled to transvenous, pericardial or epicardial leads carrying sensingand therapy delivery electrodes; leadless pacemakers, ICDs or cardiacmonitors having housing-based sensing electrodes; and external orwearable pacemakers, defibrillators, or cardiac monitors coupled toexternal, surface or skin electrodes.

FIGS. 1A and 1B are conceptual diagrams of an extra-cardiovascular ICDsystem 10 according to one example. FIG. 1A is a front view of ICDsystem 10 implanted within patient 12. FIG. 1B is a side view of ICDsystem 10 implanted within patient 12. ICD system 10 includes an ICD 14connected to an extra-cardiovascular electrical stimulation and sensinglead 16. FIGS. 1A and 1B are described in the context of an ICD system10 capable of providing defibrillation and/or cardioversion shocks andpacing pulses.

ICD 14 includes a housing 15 that forms a hermetic seal that protectsinternal components of ICD 14. The housing 15 of ICD 14 may be formed ofa conductive material, such as titanium or titanium alloy. The housing15 may function as an electrode (sometimes referred to as a “can”electrode). Housing 15 may be used as an active can electrode for use indelivering cardioversion/defibrillation (CV/DF) shocks or other highvoltage pulses delivered using a high voltage therapy circuit. In otherexamples, housing 15 may be available for use in delivering unipolar,low voltage cardiac pacing pulses and/or for sensing cardiac electricalsignals in combination with electrodes carried by lead 16. In otherinstances, the housing 15 of ICD 14 may include a plurality ofelectrodes on an outer portion of the housing. The outer portion(s) ofthe housing 15 functioning as an electrode(s) may be coated with amaterial, such as titanium nitride, e.g., for reducing post-stimulationpolarization artifact.

ICD 14 includes a connector assembly 17 (also referred to as a connectorblock or header) that includes electrical feedthroughs crossing housing15 to provide electrical connections between conductors extending withinthe lead body 18 of lead 16 and electronic components included withinthe housing 15 of ICD 14. As will be described in further detail herein,housing 15 may house one or more processors, memories, transceivers,electrical cardiac signal sensing circuitry, therapy delivery circuitry,power sources and other components for sensing cardiac electricalsignals, detecting a heart rhythm, and controlling and deliveringelectrical stimulation pulses to treat an abnormal heart rhythm.

Elongated lead body 18 has a proximal end 27 that includes a leadconnector (not shown) configured to be connected to ICD connectorassembly 17 and a distal portion 25 that includes one or moreelectrodes. In the example illustrated in FIGS. 1A and 1B, the distalportion 25 of lead body 18 includes defibrillation electrodes 24 and 26and pace/sense electrodes 28 and 30. In some cases, defibrillationelectrodes 24 and 26 may together form a defibrillation electrode inthat they may be configured to be activated concurrently. Alternatively,defibrillation electrodes 24 and 26 may form separate defibrillationelectrodes in which case each of the electrodes 24 and 26 may beactivated independently.

Electrodes 24 and 26 (and in some examples housing 15) are referred toherein as defibrillation electrodes because they are utilized,individually or collectively, for delivering high voltage stimulationtherapy (e.g., cardioversion or defibrillation shocks). Electrodes 24and 26 may be elongated coil electrodes and generally have a relativelyhigh surface area for delivering high voltage electrical stimulationpulses compared to pacing and sensing electrodes 28 and 30. However,electrodes 24 and 26 and housing 15 may also be utilized to providepacing functionality, sensing functionality or both pacing and sensingfunctionality in addition to or instead of high voltage stimulationtherapy. In this sense, the use of the term “defibrillation electrode”herein should not be considered as limiting the electrodes 24 and 26 foruse in only high voltage cardioversion/defibrillation shock therapyapplications. For example, electrodes 24 and 26 may be used as sensingelectrodes in a sensing vector for sensing cardiac electrical signalsand determining a need for an electrical stimulation therapy.

Electrodes 28 and 30 are relatively smaller surface area electrodeswhich are available for use in sensing electrode vectors for sensingcardiac electrical signals and may be used for delivering relatively lowvoltage pacing pulses in some configurations. Electrodes 28 and 30 arereferred to as pace/sense electrodes because they are generallyconfigured for use in low voltage applications, e.g., used as either acathode or anode for delivery of pacing pulses and/or sensing of cardiacelectrical signals, as opposed to delivering high voltage cardioversiondefibrillation shocks. In some instances, electrodes 28 and 30 mayprovide only pacing functionality, only sensing functionality or both.

ICD 14 may obtain cardiac electrical signals corresponding to electricalactivity of heart 8 via a combination of sensing vectors that includecombinations of electrodes 24, 26, 28 and/or 30. In some examples,housing 15 of ICD 14 is used in combination with one or more ofelectrodes 24, 26, 28 and/or 30 in a sensing electrode vector. Varioussensing electrode vectors utilizing combinations of electrodes 24, 26,28, and 30 and housing 15 are described below for acquiring first andsecond cardiac electrical signals using respective first and secondsensing electrode vectors that may be selected by sensing circuitryincluded in ICD 14.

In the example illustrated in FIGS. 1A and 1B, electrode 28 is locatedproximal to defibrillation electrode 24, and electrode 30 is locatedbetween defibrillation electrodes 24 and 26. One, two or more pace/senseelectrodes may be carried by lead body 18. For instance, a thirdpace/sense electrode may be located distal to defibrillation electrode26 in some examples. Electrodes 28 and 30 are illustrated as ringelectrodes; however, electrodes 28 and 30 may comprise any of a numberof different types of electrodes, including ring electrodes, short coilelectrodes, hemispherical electrodes, directional electrodes, segmentedelectrodes, or the like. Electrodes 28 and 30 may be positioned at anylocation along lead body 18 and are not limited to the positions shown.In other examples, lead 16 may include fewer or more pace/senseelectrodes and/or defibrillation electrodes than the example shown here.

In the example shown, lead 16 extends subcutaneously or submuscularlyover the ribcage 32 medially from the connector assembly 27 of ICD 14toward a center of the torso of patient 12, e.g., toward xiphoid process20 of patient 12. At a location near xiphoid process 20, lead 16 bendsor turns and extends superior subcutaneously or submuscularly over theribcage and/or sternum, substantially parallel to sternum 22. Althoughillustrated in FIG. 1A as being offset laterally from and extendingsubstantially parallel to sternum 22, the distal portion 25 of lead 16may be implanted at other locations, such as over sternum 22, offset tothe right or left of sternum 22, angled laterally from sternum 22 towardthe left or the right, or the like. Alternatively, lead 16 may be placedalong other subcutaneous or submuscular paths. The path ofextra-cardiovascular lead 16 may depend on the location of ICD 14, thearrangement and position of electrodes carried by the lead body 18,and/or other factors.

Electrical conductors (not illustrated) extend through one or morelumens of the elongated lead body 18 of lead 16 from the lead connectorat the proximal lead end 27 to electrodes 24, 26, 28, and 30 locatedalong the distal portion 25 of the lead body 18. The elongatedelectrical conductors contained within the lead body 18 are eachelectrically coupled with respective defibrillation electrodes 24 and 26and pace/sense electrodes 28 and 30, which may be separate respectiveinsulated conductors within the lead body 18. The respective conductorselectrically couple the electrodes 24, 26, 28, and 30 to circuitry, suchas a therapy delivery circuit and/or a sensing circuit, of ICD 14 viaconnections in the connector assembly 17, including associatedelectrical feedthroughs crossing housing 15. The electrical conductorstransmit therapy from a therapy delivery circuit within ICD 14 to one ormore of defibrillation electrodes 24 and 26 and/or pace/sense electrodes28 and 30 and transmit sensed electrical signals from one or more ofdefibrillation electrodes 24 and 26 and/or pace/sense electrodes 28 and30 to the sensing circuit within ICD 14.

The lead body 18 of lead 16 may be formed from a non-conductivematerial, including silicone, polyurethane, fluoropolymers, mixturesthereof, and other appropriate materials, and shaped to form one or morelumens within which the one or more conductors extend. Lead body 18 maybe tubular or cylindrical in shape. In other examples, the distalportion 25 (or all of) the elongated lead body 18 may have a flat,ribbon or paddle shape. Lead body 18 may be formed having a preformeddistal portion 25 that is generally straight, curving, bending,serpentine, undulating or zig-zagging.

In the example shown, lead body 18 includes a curving distal portion 25having two “C” shaped curves, which together may resemble the Greekletter epsilon, “ε.” Defibrillation electrodes 24 and 26 are eachcarried by one of the two respective C-shaped portions of the lead bodydistal portion 25. The two C-shaped curves are seen to extend or curvein the same direction away from a central axis of lead body 18, alongwhich pace/sense electrodes 28 and 30 are positioned. Pace/senseelectrodes 28 and 30 may, in some instances, be approximately alignedwith the central axis of the straight, proximal portion of lead body 18such that mid-points of defibrillation electrodes 24 and 26 arelaterally offset from pace/sense electrodes 28 and 30.

Other examples of extra-cardiovascular leads including one or moredefibrillation electrodes and one or more pacing and sensing electrodescarried by curving, serpentine, undulating or zig-zagging distal portionof the lead body 18 that may be implemented with the techniquesdescribed herein are generally disclosed in pending U.S. Pat.Publication No. 2016/0158567 (Marshall, et al.), incorporated herein byreference in its entirety. The techniques disclosed herein are notlimited to any particular lead body design, however. In other examples,lead body 18 is a flexible elongated lead body without any pre-formedshape, bends or curves. Various example configurations ofextra-cardiovascular leads and electrodes and dimensions that may beimplemented in conjunction with the cardiac event sensing techniquesdisclosed herein are described in pending U.S. Publication No.2015/0306375 (Marshall, et al.) and pending U.S. Publication No.2015/0306410 (Marshall, et al.), both of which are incorporated hereinby reference in their entirety.

ICD 14 analyzes the cardiac electrical signals received from one or moresensing electrode vectors to monitor for abnormal rhythms, such asbradycardia, ventricular tachycardia (VT) or ventricular fibrillation(VF). ICD 14 may analyze the heart rate and morphology of the cardiacelectrical signals to monitor for tachyarrhythmia in accordance with anyof a number of tachyarrhythmia detection techniques. One exampletechnique for detecting tachyarrhythmia is described in U.S. Pat. No.7,761,150 (Ghanem, et al.), incorporated herein by reference in itsentirety.

ICD 14 generates and delivers electrical stimulation therapy in responseto detecting a tachyarrhythmia (e.g., VT or VF) using a therapy deliveryelectrode vector which may be selected from any of the availableelectrodes 24, 26, 28 30 and/or housing 15. ICD 14 may deliveranti-tachycardia pacing (ATP) in response to VT detection, and in somecases may deliver ATP prior to a CV/DF shock or during high voltagecapacitor charging in an attempt to avert the need for delivering aCV/DF shock. If ATP does not successfully terminate VT or when VF isdetected, ICD 14 may deliver one or more CV/DF shocks via one or both ofdefibrillation electrodes 24 and 26 and/or housing 15. ICD 14 maydeliver the CV/DF shocks using electrodes 24 and 26 individually ortogether as a cathode (or anode) and with the housing 15 as an anode (orcathode). ICD 14 may generate and deliver other types of electricalstimulation pulses such as post-shock pacing pulses or bradycardiapacing pulses using a pacing electrode vector that includes one or moreof the electrodes 24, 26, 28, and 30 and the housing 15 of ICD 14.

FIGS. 1A and 1B are illustrative in nature and should not be consideredlimiting of the practice of the techniques disclosed herein. ICD 14 isshown implanted subcutaneously on the left side of patient 12 along theribcage 32. ICD 14 may, in some instances, be implanted between the leftposterior axillary line and the left anterior axillary line of patient12. ICD 14 may, however, be implanted at other subcutaneous orsubmuscular locations in patient 12. For example, ICD 14 may beimplanted in a subcutaneous pocket in the pectoral region. In this case,lead 16 may extend subcutaneously or submuscularly from ICD 14 towardthe manubrium of sternum 22 and bend or turn and extend inferiorly fromthe manubrium to the desired location subcutaneously or submuscularly.In yet another example, ICD 14 may be placed abdominally. Lead 16 may beimplanted in other extra-cardiovascular locations as well. For instance,as described with respect to FIGS. 2A-2C, the distal portion 25 of lead16 may be implanted underneath the sternum/ribcage in the substernalspace.

An external device 40 is shown in telemetric communication with ICD 14by a communication link 42. External device 40 may include a processor52, memory 53, display 54, user interface 56 and telemetry unit 58.Processor 52 controls external device operations and processes data andsignals received from ICD 14. Display 54, which may include a graphicaluser interface, displays data and other information to a user forreviewing ICD operation and programmed parameters as well as cardiacelectrical signals retrieved from ICD 14. For example, as described inconjunction with FIG. 5, a clinician may view cardiac electrical signalsreceived from ICD 14 during a slow, non-paced rhythm for establishingreference P-wave signal amplitudes used by ICD 14 for detecting P-waveoversensing.

User interface 56 may include a mouse, touch screen, key pad or the liketo enable a user to interact with external device 40 to initiate atelemetry session with ICD 14 for retrieving data from and/ortransmitting data to ICD 14, including programmable parameters forcontrolling cardiac event sensing and therapy delivery. Telemetry unit58 includes a transceiver and antenna configured for bidirectionalcommunication with a telemetry circuit included in ICD 14 and isconfigured to operate in conjunction with processor 52 for sending andreceiving data relating to ICD functions via communication link 42.

Communication link 42 may be established between ICD 14 and externaldevice 40 using a radio frequency (RF) link such as BLUETOOTH®, Wi-Fi,or Medical Implant Communication Service (MICS) or other RF orcommunication frequency bandwidth or communication protocols. Datastored or acquired by ICD 14, including physiological signals orassociated data derived therefrom, results of device diagnostics, andhistories of detected rhythm episodes and delivered therapies, may beretrieved from ICD 14 by external device 40 following an interrogationcommand.

External device 40 may be embodied as a programmer used in a hospital,clinic or physician's office to retrieve data from ICD 14 and to programoperating parameters and algorithms in ICD 14 for controlling ICDfunctions. External device 40 may alternatively be embodied as a homemonitor or hand held device. External device 40 may be used to programcardiac signal sensing parameters, cardiac rhythm detection parametersand therapy control parameters used by ICD 14. At least some controlparameters used in detecting P-wave oversensing according to techniquesdisclosed herein may be programmed into ICD 14 using external device 40in some examples.

FIGS. 2A-2C are conceptual diagrams of patient 12 implanted withextra-cardiovascular ICD system 10 in a different implant configurationthan the arrangement shown in FIGS. 1A-1B. FIG. 2A is a front view ofpatient 12 implanted with ICD system 10. FIG. 2B is a side view ofpatient 12 implanted with ICD system 10. FIG. 2C is a transverse view ofpatient 12 implanted with ICD system 10. In this arrangement,extra-cardiovascular lead 16 of system 10 is implanted at leastpartially underneath sternum 22 of patient 12. Lead 16 extendssubcutaneously or submuscularly from ICD 14 toward xiphoid process 20and at a location near xiphoid process 20 bends or turns and extendssuperiorly within anterior mediastinum 36 in a substernal position.

Anterior mediastinum 36 may be viewed as being bounded laterally bypleurae 39, posteriorly by pericardium 38, and anteriorly by sternum 22(see FIG. 2C). The distal portion 25 of lead 16 may extend along theposterior side of sternum 22 substantially within the loose connectivetissue and/or substernal musculature of anterior mediastinum 36. A leadimplanted such that the distal portion 25 is substantially withinanterior mediastinum 36, may be referred to as a “substernal lead.”

In the example illustrated in FIGS. 2A-2C, lead 16 is locatedsubstantially centered under sternum 22. In other instances, however,lead 16 may be implanted such that it is offset laterally from thecenter of sternum 22. In some instances, lead 16 may extend laterallysuch that distal portion 25 of lead 16 is underneath/below the ribcage32 in addition to or instead of sternum 22. In other examples, thedistal portion 25 of lead 16 may be implanted in otherextra-cardiovascular, intra-thoracic locations, including the pleuralcavity or around the perimeter of and adjacent to but typically notwithin the pericardium 38 of heart 8. Other implant locations and leadand electrode arrangements that may be used in conjunction with thecardiac event sensing techniques described herein are generallydisclosed in the above-incorporated references.

FIG. 3 is a schematic diagram of ICD 14 according to one example. Theelectronic circuitry enclosed within housing 15 (shown schematically asan electrode in FIG. 3) includes software, firmware and hardware thatcooperatively monitor cardiac electrical signals, determine when anelectrical stimulation therapy is necessary, and deliver therapy asneeded according to programmed therapy delivery algorithms and controlparameters. ICD 14 is coupled to an extra-cardiovascular lead, such aslead 16 carrying extra-cardiovascular electrodes 24, 26, 28, and 30, fordelivering electrical stimulation pulses to the patient's heart and forsensing cardiac electrical signals.

ICD 14 includes a control circuit 80, memory 82, therapy deliverycircuit 84, sensing circuit 86, and telemetry circuit 88. A power source98 provides power to the circuitry of ICD 14, including each of thecomponents 80, 82, 84, 86, and 88 as needed. Power source 98 may includeone or more energy storage devices, such as one or more rechargeable ornon-rechargeable batteries. The connections between power source 98 andeach of the other components 80, 82, 84, 86 and 88 are to be understoodfrom the general block diagram of FIG. 3, but are not shown for the sakeof clarity. For example, power source 98 may be coupled to one or morecharging circuits included in therapy delivery circuit 84 for chargingholding capacitors included in therapy delivery circuit 84 that aredischarged at appropriate times under the control of control circuit 80for producing electrical pulses according to a therapy protocol, such asfor ventricular pacing during atrio-ventricular conduction block orbradycardia, post-shock pacing, ATP and/or CV/DF shock pulses. Powersource 98 is also coupled to components of sensing circuit 86, such assense amplifiers, analog-to-digital converters, switching circuitry,etc. as needed.

The circuits shown in FIG. 3 represent functionality included in ICD 14and may include any discrete and/or integrated electronic circuitcomponents that implement analog and/or digital circuits capable ofproducing the functions attributed to ICD 14 herein. Functionalityassociated with one or more circuits may be performed by separatehardware, firmware or software components, or integrated within commonhardware, firmware or software components. For example, cardiac eventsensing may be performed cooperatively by sensing circuit 86 and controlcircuit 80 and may include operations implemented in a processor orother signal processing circuitry included in control circuit 80executing instructions stored in memory 82 and control signals such asblanking and timing intervals and sensing threshold amplitude signalssent from control circuit 80 to sensing circuit 86. The various circuitsof ICD 14 may include an application specific integrated circuit (ASIC),an electronic circuit, a processor (shared, dedicated, or group) andmemory that execute one or more software or firmware programs, acombinational logic circuit, state machine, or other suitable componentsor combinations of components that provide the described functionality.The particular form of software, hardware and/or firmware employed toimplement the functionality disclosed herein will be determinedprimarily by the particular system architecture employed in an ICD orpacemaker and by the particular detection and therapy deliverymethodologies employed by the ICD or pacemaker. Providing software,hardware, and/or firmware to accomplish the described functionality inthe context of any modern implantable cardiac device system, given thedisclosure herein, is within the abilities of one of skill in the art.

Memory 82 may include any volatile, non-volatile, magnetic, orelectrical non-transitory computer readable storage media, such asrandom access memory (RAM), read-only memory (ROM), non-volatile RAM(NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory,or any other memory device. Furthermore, memory 82 may includenon-transitory computer readable media storing instructions that, whenexecuted by one or more processing circuits, cause control circuit 80and/or other ICD components to perform various functions attributed toICD 14 or those ICD components. The non-transitory computer-readablemedia storing the instructions may include any of the media listedabove.

Control circuit 80 communicates, e.g., via a data bus, with therapydelivery circuit 84 and sensing circuit 86 for sensing cardiacelectrical activity, detecting cardiac rhythms, and controlling deliveryof cardiac electrical stimulation therapies in response to sensedcardiac signals. Therapy delivery circuit 84 and sensing circuit 86 areelectrically coupled to electrodes 24, 26, 28, 30 carried by lead 16 andthe housing 15, which may function as a common or ground electrode or asan active can electrode for delivering CV/DF shock pulses or cardiacpacing pulses.

Sensing circuit 86 may be selectively coupled to electrodes 28, 30and/or housing 15 in order to monitor electrical activity of thepatient's heart. Sensing circuit 86 may additionally be selectivelycoupled to defibrillation electrodes 24 and/or 26 for use in a sensingelectrode vector together or in combination with one or more ofelectrodes 28, 30 and/or housing 15. Sensing circuit 86 may be enabledto selectively receive cardiac electrical signals from at least twosensing electrode vectors from the available electrodes 24, 26, 28, 30,and housing 15. At least two cardiac electrical signals from twodifferent sensing electrode vectors may be received simultaneously bysensing circuit 86. Sensing circuit 86 may monitor one or both of thecardiac electrical signals simultaneously for sensing cardiac electricalevents and producing digitized cardiac signal waveforms for analysis bycontrol circuit 80. For example, sensing circuit 86 may includeswitching circuitry for selecting which of electrodes 24, 26, 28, 30,and housing 15 are coupled to a first sensing channel 83 and which arecoupled to a second sensing channel 85 of sensing circuit 86.

Each sensing channel 83 and 85 may be configured to amplify, filter anddigitize the cardiac electrical signal received from selected electrodescoupled to the respective sensing channel to improve the signal qualityfor detecting cardiac electrical events, such as R-waves or performingother signal analysis. The cardiac event detection circuitry withinsensing circuit 86 may include one or more sense amplifiers, filters,rectifiers, threshold detectors, comparators, analog-to-digitalconverters (ADCs), timers or other analog or digital components asdescribed further in conjunction with FIG. 4. A cardiac event sensingthreshold may be automatically adjusted by sensing circuit 86 under thecontrol of control circuit 80, based on timing intervals and sensingthreshold values determined by control circuit 80, stored in memory 82,and/or controlled by hardware, firmware and/or software of controlcircuit 80 and/or sensing circuit 86.

Upon detecting a cardiac electrical event based on a sensing thresholdcrossing by a received cardiac signal, sensing circuit 86 may produce asensed event signal, such as an R-wave sensed event signal, that ispassed to control circuit 80. The R-wave sensed event signals may beused by control circuit 80 to reset a pacing escape interval used tocontrol the basic timing of pacing pulses generated by therapy deliverycircuit 84. R-wave sensed event signals may also be used by controlcircuit 80 for determining RR intervals (RRIs) for detectingtachyarrhythmia and determining a need for therapy. An RRI is the timeinterval between two consecutively sensed R-waves and may be determinedbetween two consecutive R-wave sensed event signals received fromsensing circuit 86.

Control circuit 80 may include comparators and counters for countingRRIs that fall into various rate detection zones for determining aventricular rate or performing other rate- or interval-based assessmentsfor detecting VT and VF. For example, control circuit 80 may compare thedetermined RRIs to one or more tachyarrhythmia detection interval zones,such as a tachycardia detection interval zone and a fibrillationdetection interval zone. RRIs falling into a detection interval zone arecounted by a respective VT interval counter or VF interval counter andin some cases in a combined VT/VF interval counter included in controlcircuit 80. When a VT or VF interval counter reaches a threshold countvalue, often referred to as “number of intervals to detect” or “NID,” aventricular tachyarrhythmia may be detected by control circuit 80.

To support additional cardiac signal analyses performed control circuit80, sensing circuit 86 may pass a digitized cardiac electrical signal tocontrol circuit 80. A cardiac electrical signal from the selectedsensing channel, e.g., from first sensing channel 83 and/or the secondsensing channel 85, may be passed through a filter and amplifier,provided to a multiplexer and thereafter converted to multi-bit digitalsignals by an analog-to-digital converter, all included in sensingcircuit 86, for storage in memory 82.

Therapy delivery circuit 84 includes charging circuitry, one or morecharge storage devices such as one or more high voltage capacitorsand/or low voltage capacitors, and switching circuitry that controlswhen the capacitor(s) are discharged across a selected pacing electrodevector or CV/DF shock vector. Charging of capacitors to a programmedpulse amplitude and discharging of the capacitors for a programmed pulsewidth may be performed by therapy delivery circuit 84 according tocontrol signals received from control circuit 80. Control circuit 80 mayinclude various timers or counters that control when cardiac pacingpulses are delivered. For example, a timing control circuit included incontrol circuit 80 may include programmable digital counters set by amicroprocessor of the control circuit 80 for controlling the basicpacing time intervals associated with various pacing modes or ATPsequences delivered by ICD 14. The microprocessor of control circuit 80may also set the amplitude, pulse width, polarity or othercharacteristics of the cardiac pacing pulses, which may be based onprogrammed values stored in memory 82.

In response to detecting VT or VF, control circuit 80 may controltherapy delivery circuit 84 to deliver therapies such as ATP and/orCV/DF therapy. Therapy can be delivered by initiating charging of highvoltage capacitors via a charging circuit, both included in therapydelivery circuit 84. Charging is controlled by control circuit 80 whichmonitors the voltage on the high voltage capacitors, which is passed tocontrol circuit 80 via a charging control line. When the voltage reachesa predetermined value set by control circuit 80, a logic signal isgenerated on a capacitor full line and passed to therapy deliverycircuit 84, terminating charging. A CV/DF pulse is delivered to theheart under the control of the timing circuit 90 by an output circuit oftherapy delivery circuit 84 via a control bus. The output circuit mayinclude an output capacitor through which the charged high voltagecapacitor is discharged via switching circuitry, e.g., an H-bridge,which determines the electrodes used for delivering the cardioversion ordefibrillation pulse and the pulse wave shape.

In some examples, the high voltage therapy circuit configured to deliverCV/DF shock pulses can be controlled by control circuit 80 to deliverpacing pulses, e.g., for delivering ATP, post shock pacing pulses orventricular pacing pulses during atrio-ventricular conduction block orbradycardia. In other examples, therapy delivery circuit 84 may includea low voltage therapy circuit for generating and delivering pacingpulses for a variety of pacing needs. Therapy delivery and controlcircuitry generally disclosed in any of the incorporated references maybe implemented in ICD 14.

It is recognized that the methods disclosed herein may be implemented inan IMD that is used for monitoring cardiac electrical signals by sensingcircuit 86 and control circuit 80 without having therapy deliverycapabilities or in an IMD that monitors cardiac electrical signals anddelivers cardiac pacing therapies by therapy delivery circuit 84,without high voltage therapy capabilities, such ascardioversion/defibrillation shock capabilities or vice versa.

Control parameters utilized by control circuit 80 for sensing cardiacevents and controlling therapy delivery may be programmed into memory 82via telemetry circuit 88. Telemetry circuit 88 includes a transceiverand antenna for communicating with external device 40 (shown in FIG. 1A)using RF communication or other communication protocols as describedabove. Under the control of control circuit 80, telemetry circuit 88 mayreceive downlink telemetry from and send uplink telemetry to externaldevice 40. In some cases, telemetry circuit 88 may be used to transmitand receive communication signals to/from another medical deviceimplanted in patient 12.

FIG. 4 is a diagram of circuitry included in sensing circuit 86 havingfirst sensing channel 83 and second sensing channel 85 according to oneexample. Switching circuitry 61 may include a switch array, switchmatrix, multiplexer, or any other type of switching device suitable toselectively couple first and second sensing channels 83 and 85 torespective sensing electrode vectors. First sensing channel 83 may beselectively coupled via switching circuitry 61 to a first sensingelectrode vector including at least one electrode carried byextra-cardiovascular lead 16 as shown in FIGS. 1A-2C for receiving afirst cardiac electrical signal. First sensing channel 83 may be coupledto a sensing electrode vector that is a short bipole, having arelatively shorter inter-electrode distance or spacing than the secondelectrode vector coupled to second sensing channel 85. First sensingchannel 83 may be coupled to a sensing electrode vector that isapproximately vertical (when the patient is in an upright position) orapproximately aligned with the cardiac axis to increase the likelihoodof a relatively high R-wave signal amplitude relative to P-wave signalamplitude. In one example, the first sensing electrode vector mayinclude pace/sense electrodes 28 and 30. In other examples, the firstsensing electrode vector coupled to sensing channel 83 may include adefibrillation electrode 24 and/or 26, e.g., a sensing electrode vectorbetween pace/sense electrode 28 and defibrillation electrode 24 orbetween pace/sense electrode 30 and either of defibrillation electrodes24 or 26. In still other examples, the first sensing electrode vectormay be between defibrillation electrodes 24 and 26.

Sensing circuit 86 includes second sensing channel 85 that receives asecond cardiac electrical signal from a second sensing vector, forexample from a vector that includes one electrode 24, 26, 28 or 30carried by lead 16 paired with housing 15. Second sensing channel 85 maybe selectively coupled to other sensing electrode vectors, which mayform a relatively longer bipole having an inter-electrode distance orspacing that is greater than the sensing electrode vector coupled tofirst sensing channel 83 in some examples and may be approximatelyorthogonal to the first channel sensing electrode vector in some cases.For instance, defibrillation electrodes 26 and housing 15 may be coupledto second sensing channel 85 to provide the second cardiac electricalsignal. As described below, the second cardiac electrical signalreceived by second sensing channel 85 via a long bipole may be used bycontrol circuit 80 for comparative analysis of sensed cardiac event peakamplitudes for detecting P-wave oversensing. The long bipole coupled tosecond sensing channel 85 may provide a far-field signal compared to therelatively shorter bipole coupled to the first sensing channel. In therelatively more far-field signal, the amplitude of P-waves may berelatively higher than in the more near-field signal. In other examples,any vector selected from the available electrodes, e.g., electrodes 24,26, 28, 30 and/or housing 15, may be included in a sensing electrodevector coupled to second sensing channel 85. The sensing electrodevectors coupled to first sensing channel 83 and second sensing channel85 are different sensing electrode vectors, which may have no commonelectrodes or only one common electrode but not both.

In the illustrative example shown in FIG. 4, the electrical signalsdeveloped across the first sensing electrode vector, e.g., electrodes 28and 30, are received by first sensing channel 83 and electrical signalsdeveloped across the second sensing electrode vector, e.g., electrodes26 and housing 15, are received by second sensing channel 85. Thecardiac electrical signals are provided as differential input signals tothe pre-filter and pre-amplifier 62 or 72, respectively, of firstsensing channel 83 and second sensing channel 85. Non-physiological highfrequency and DC signals may be filtered by a low pass or bandpassfilter included in each of pre-filter and pre-amplifiers 62 and 72, andhigh voltage signals may be removed by protection diodes included inpre-filter and pre-amplifiers 62 and 72. Pre-filter and pre-amplifiers62 and 72 may amplify the pre-filtered signal by a gain of between 10and 100, and in one example a gain of 17.5, and may convert thedifferential signal to a single-ended output signal passed toanalog-to-digital converter (ADC) 63 in first sensing channel 83 and toADC 73 in second sensing channel 85. Pre-filters and amplifiers 62 and72 may provide anti-alias filtering and noise reduction prior todigitization.

ADC 63 and ADC 73, respectively, convert the first cardiac electricalsignal from an analog signal to a first digital bit stream and thesecond cardiac electrical signal to a second digital bit stream. In oneexample, ADC 63 and ADC 73 may be sigma-delta converters (SDC), butother types of ADCs may be used. In some examples, the outputs of ADC 63and ADC 73 may be provided to decimators (not shown), which function asdigital low-pass filters that increase the resolution and reduce thesampling rate of the respective first and second cardiac electricalsignals.

The digital outputs of ADC 63 and ADC 73 are each passed to respectivefilters 64 and 74, which may be digital bandpass filters. The bandpassfilters 64 and 74 may have the same or different bandpass frequencies.For example, filter 64 may have a bandpass of approximately 13 Hz to 39Hz for passing cardiac electrical signals such as R-waves typicallyoccurring in this frequency range. Filter 74 of the second sensingchannel 85 may have a bandpass of approximately 2.5 to 100 Hz. In someexamples, second sensing channel 85 may further include a notch filterto filter 60 Hz or 50 Hz noise signals.

The bandpass filtered signals are passed from filters 64 and 74 torectifiers 65 and 75, respectively to produce a filtered, rectifiedsignal. First sensing channel 83 includes an R-wave detector 66 forsensing cardiac events in response to the first cardiac electricalsignal crossing an R-wave sensing threshold. The second sensing channel85 may optionally include an R-wave detector 76 as shown. In otherexamples, the second sensing channel 85 does not include R-wave detector76. The filtered, rectified digital cardiac electrical signals 69 and 79from sensing channel 83 and sensing channel 85 may be passed to controlcircuit 80 for use in detecting P-wave oversensing as described below inconjunction with FIGS. 5-7. For instance, control circuit 80 isconfigured to determine a peak amplitude of at least one of thedigitized cardiac electrical signals 69 or 79 following an R-wave sensedevent signal for use in detecting P-wave oversensing as described below.The digital signals 69 and 79 may also be used by control circuit 80 indetecting and discriminating tachyarrhythmia episodes.

R-wave detector 66 (and 76 if included) may include an auto-adjustingsense amplifier, comparator and/or other detection circuitry thatcompares the filtered and rectified cardiac electrical signal to anR-wave sensing threshold in real time and produces an R-wave sensedevent signal 68 (and/or 78) when the cardiac electrical signal crossesthe R-wave sensing threshold outside of a post-sense blanking interval.

The R-wave sensing threshold may be a multi-level sensing threshold asdisclosed in pending U.S. patent application Ser. No. 15/142,171 (Cao,et al., filed on Apr. 29, 2016), incorporated herein by reference in itsentirety. Briefly, the multi-level sensing threshold may have a startingsensing threshold value held for a time interval, which may be equal toa tachycardia detection interval or expected R-wave to T-wave interval,then drops to a second sensing threshold value held until a drop timeinterval expires, which may be 1 to 2 seconds long. The sensingthreshold drops to a minimum sensing threshold, which may correspond toa programmed sensitivity sometimes referred to as the “sensing floor”,after the drop time interval. In other examples, the R-wave sensingthreshold used by R-wave detector 66 may be set to a starting valuebased on the peak amplitude determined during the most recent post-senseblanking interval and decay linearly or exponentially over time untilreaching a minimum sensing threshold. The techniques described hereinare not limited to a specific behavior of the sensing threshold.Instead, other decaying, step-wise adjusted or other automaticallyadjusted sensing thresholds may be utilized.

The configuration of sensing channels 83 and 85 shown in FIG. 4 isillustrative in nature and should not be considered limiting of thetechniques described herein. The sensing channels 83 and 85 of sensingcircuit 86 may include more or fewer components than illustrated anddescribed in FIG. 4.

FIG. 5 is a diagram of two cardiac electrical signals 100 and 110provided by the first sensing channel 83 and second sensing channel 85,respectively, of sensing circuit 86. The signals 100 and 110 are shownprior to rectification by rectifiers 65 and 75. The first sensingchannel 83 may sense each R-wave 102 and produce an R-wave sensed eventsignal 108 in response to cardiac electrical signal 100 crossing anR-wave sensing threshold 120. R-wave sensing threshold 120 is shown as amulti-level threshold which may be automatically adjusted to differentsensing threshold values at specified time intervals by sensing circuit86 under the control of control circuit 80.

Control circuit 80 may be configured to establish a reference peakamplitude of the P-waves 106 and 116 for each sensing channel 83 and 85,respectively. Control circuit 80 may buffer each cardiac electricalsignal 100 and 110 over one or more cardiac cycles, e.g., from oneR-wave sensed event signal to the next. Control circuit 80 may set aP-wave window 104 at a predetermined time interval earlier than a givenR-wave sensed event signal 108. P-wave window 104 may extend up to 300ms earlier than the R-wave sensed event signal 108. In one example,P-wave window 104 begins 200 ms earlier than the R-wave sensed eventsignal 108 and is 100 to 150 ms long. In some examples, the P-wavewindow 104 may be adjusted manually by a clinician interacting withexternal device 40. For instance, signals 100 and 110 may be transmittedfrom ICD 14 to external device 40 and displayed on display 54 (FIG. 1A).The clinician may adjust the starting time, ending time and/or width(duration) of P-wave window 104. The timing of the manually adjustedP-wave window may be transmitted to ICD 14 from external device 40 andused for setting P-wave windows relative to R-wave sensed event signals.The starting, ending and duration of P-wave window 104 may be stored inmemory 82 for use by control circuit 80 in automatically updatingreference P-wave amplitude values during a resting (slow) non-pacedheart rhythm.

The P-wave window 104 is applied to both the first cardiac electricalsignal 100 and the second cardiac electrical signal 110. The maximumpeak amplitude during the P-wave window 104 is determined from the firstsensing channel signal 100 and from the second sensing channel signal110 to obtain a reference peak amplitude for each of the sensingchannels 100 and 110, respectively. While non-rectified signals 100 and110 are shown in FIG. 5, it is to be understood that the peak amplitudeswithin P-wave window 104 may be determined from the filtered, rectifiedsignal outputs 69 and 79 of the respective sensing channels 83 and 85.

In the example shown, the second cardiac electrical signal 110 hasrelatively higher amplitude P-waves 116 than the relatively smallP-waves 106 of the first cardiac signal 100. This relative difference inP-wave amplitude between the first and second signals 100 and 110 of therespective first and second sensing channels 83 and 85 may be used indetecting P-wave oversensing. When the first sensing channel 83 iscoupled to a relatively shorter bipole that is relatively more proximateto the patient's ventricular chambers and/or more aligned with thecardiac axis than the sensing vector coupled to the second cardiacchannel 85, the cardiac signal 100 is expected to include relativelyhigher amplitude R-waves and lower amplitude P-waves. If the secondsensing channel 85 is coupled to a relatively long bipole that is lessproximate to the ventricular chambers and/or closer to atrial chambers,cardiac signal 110 is expected to have relatively larger P-waves 116than the first cardiac signal 100. The relative differences betweenP-wave amplitudes 106 and 116 and/or the reference values of P-waveamplitudes 106 and 116 may be used for detecting P-wave oversensing asdescribed below.

FIG. 6 is a flow chart 200 of a method performed by ICD 14 for detectingP-wave oversensing (PWOS) according to one example. At block 201,control circuit 80 may determine reference P-wave peak amplitudes fromeach of the two cardiac electrical signals received by sensing circuit86. The reference peak amplitudes may be determined during a slow (e.g.,resting), intrinsic (non-paced) rhythm as described above in conjunctionwith FIG. 5.

At block 202, control circuit 80 may start a pacing interval controlledby a timer or counter included in control circuit 80. The pacinginterval is started upon a cardiac event, which may be a deliveredpacing pulse or an initially sensed intrinsic cardiac event. At block204, the first sensing channel 83 senses a cardiac event during thepacing interval based on an R-wave sensing threshold crossing by thefirst cardiac electrical signal. The first sensing channel 83 mayproduce an R-wave sensed event signal that is passed to control circuit80. Control circuit 80 starts a post-sense blanking interval at block205. The post-sense blanking interval may be 100 to 200 ms long, e.g.,150 ms. The post-sense blanking interval avoids double sensing of asingle R-wave and oversensing of non-cardiac noise within aphysiological refractory period of the myocardium.

Control circuit 80 is configured to detect event intervals that aregreater than a P-wave oversensing (PWOS) threshold interval. Controlcircuit 80 determines the cardiac event interval since the most recentpreceding cardiac event (sensed or paced) to the current R-wave sensedevent signal at block 206. The cardiac event interval ending with thecurrently sensed event is compared to the PWOS threshold interval atblock 206. In order to detect the currently sensed event as a PWOSevent, the cardiac event interval ending with the currently sensed eventmay be required to be greater than the PWOS threshold interval, e.g.,greater than 600 ms. The PWOS threshold interval may be greater than 400ms in other examples.

The PWOS threshold interval may be set by control circuit 80 to begreater than a tachyarrhythmia detection interval to prevent a lowamplitude R-wave during a ventricular tachyarrhythmia from being falselydetected as PWOS, potentially delaying or preventing an appropriate VTor VF detection. During relatively fast rhythms, cardiac pacing to treatasystole or bradycardia is not needed. As such, PWOS detection forcontrolling pacing timing intervals during fast rhythms, e.g., eventintervals shorter than 400 to 600 ms, may not be needed. During a slowrhythm, undetected PWOS may interfere with pacing delivery if a P-waveis falsely sensed as an R-wave, causing a pacing pulse to be inhibited.As such, the requirement of the cardiac event interval being greaterthan a PWOS threshold interval enables PWOS to be detected when thecardiac event interval is longer than the longest tachyarrhythmiadetection interval such that the rate of sensed events is relativelyslow and the potential need for cardiac pacing exists. The PWOSthreshold interval requirement for detecting PWOS avoids false PWOSdetection during fast rhythms which could interfere with tachyarrhythmiadetection.

The PWOS threshold interval may be a fixed interval, which may beprogrammable by a user, or a variable interval that is set by controlcircuit 80 based on the programmed pacing rate and/or the longesttachyarrhythmia detection interval. The PWOS threshold interval may begreater than the longest tachyarrhythmia detection interval, which maybe 400 ms or less, and less than the pacing interval corresponding tothe programmed lower pacing rate. If the programmed pacing rate is 40pulses per minute, the pacing rate interval is 1500 ms. The PWOSthreshold interval may be set by control circuit to half of this pacinginterval or 750 ms. In other examples, the PWOS threshold interval maybe set to 40% to 70% of the pacing interval corresponding to theprogrammed lower rate (which may typically be 40 to 60 pulses per minutein most patients) but not less than the longest tachyarrhythmiadetection interval.

If the cardiac event interval is not greater than the PWOS threshold atblock 206, control circuit 80 may restart the pacing interval timer orcounter at block 218. A scheduled pacing pulse is inhibited in responseto the sensed event. PWOS is not detected.

If a cardiac event interval greater than the PWOS threshold interval isdetected at block 206, control circuit 80 determines the peak amplitudeof the first cardiac electrical signal at block 208. The peak amplitudeis determined from the first sensing channel 83 during the post senseblanking interval (started at block 205). This peak amplitude may bereferred to as P1. At block 208, the control circuit 80 may alsodetermine the peak amplitude of the second cardiac electrical signalreceived from the second sensing channel 85 during the post-senseblanking interval. This peak amplitude may be referred to as P2.

The illustrative examples presented herein utilize the peak amplitudesof the first and second cardiac signals determined during the post-senseblanking interval. It is contemplated that other signal features may bedetermined, such as a signal width, slope, or area. The signal area maybe determined by summing the amplitudes of each sample point of thesignal pulse having the maximum peak amplitude during the post-senseblanking interval. In another example, the average signal width may bedetermined as the signal area divided by the maximum peak amplitude. Itis understood that for each signal feature determined during thepost-sense blanking interval for use in detecting PWOS, a referencevalue for the analogous signal feature is previously established bydetermining the analogous signal feature during P-wave window 104 (FIG.5) during a normal, slow, non-paced heart rhythm.

Referring again to FIG. 6, the peak amplitudes P1 and P2 determined fromthe first and second cardiac electrical signals, respectively, may becompared to each other at block 210. If P1 is not less than P2, thesensed event may not be a P-wave. PWOS criteria may be unsatisfied if P1is greater than P2 in some examples. PWOS is not detected, and controlcircuit 80 may restart the pacing interval at block 218 in response tothe currently sensed event.

If P1 is less than P2, as expected based on the characteristics ofP-waves in the first and second cardiac electrical signals as shown inFIG. 5, the sensed event has a higher likelihood of being an oversensedP-wave. At block 212, control circuit 80 may compare the peak amplitudesP1 and P2 to the respective reference P-wave peak amplitudes previouslyestablished for the first channel 83 and the second channel 85 at block202. Control circuit 80 may compare the peak amplitude P1 to itsrespective reference value by determining the absolute differencebetween peak amplitude P1 and the reference value determined for thefirst sensing channel 83. If the absolute difference is less than athreshold percentage of the reference peak amplitude, the currentlysensed event may be an oversensed P-wave.

Similarly, the peak amplitude P2 may be compared to its reference peakamplitude value by determining if the absolute difference between P2 andthe reference P-wave peak amplitude determined for the second sensingchannel 85 is less than a threshold percentage. If both P1 and P2 arewithin a threshold percentage of their respective reference values, thesensed event is likely an oversensed P-wave given that P1 is less thanP2. The threshold percentage may be 100% or less. The P-wave amplitudeis not expected to change from its reference value by more than 100%.Sensed events that have a peak amplitude that represents more than a100% (or other threshold) change in amplitude from the respectivereference P-wave peak amplitude may be true R-waves, in which case thepacing interval should be reset so that the scheduled pacing pulse isinhibited.

If the P1 and P2 amplitudes are more than the threshold difference fromtheir respective references amplitudes, the scheduled pacing pulse isinhibited by restarting the pacing interval at block 218. Controlcircuit 80 returns to block 204 to wait for the next sensed event. Ifthe P1 and P2 amplitudes are both within the threshold difference fromthe reference P-wave peak amplitudes, “yes” branch of block 212, PWOS isdetected at block 214. Control circuit 80 withholds restarting thepacing interval at block 216. The sensed event identified as a PWOSevent does not cause the control circuit 80 to inhibit a scheduledpacing pulse. Control circuit 80 allows the pacing interval to continuerunning and returns to block 204 to wait for the next sensed event.

In this way, PWOS is detected on an event-by-event basis for use incontrolling the pacing timing interval on each cardiac cycle to properlyinhibit or deliver a pacing pulse as needed. While not shown in FIG. 6,it is to be understood that if the pacing interval expires before anevent is sensed at block 204 that is not detected as PWOS, controlcircuit 80 controls therapy delivery circuit 84 to deliver a pacingpulse. The pacing interval is restarted in response to the deliveredpacing pulse. In this way, the heart rate does not fall below thecorresponding pacing lower rate even in the presence of PWOS.

According to the flow chart 200 of FIG. 6, PWOS detection criteriaincludes a requirement of the cardiac event interval ending with thecurrently sensed event being greater than the PWOS threshold interval,P1 being greater than P2, and P1 and P2 both being within apredetermined threshold difference from their respective referenceP-wave peak amplitude values. It is to be understood that in otherexamples, any combination of these criteria may be used to identify thecurrently sensed event as a PWOS event. For instance, the cardiac eventinterval may be required to be greater than a PWOS threshold intervaland at least P1 may be required to be within a threshold difference ofthe reference P-wave peak amplitude determined for the first sensingchannel 83. In another example, the cardiac event interval may berequired to be greater than the PWOS threshold interval and both P1 andP2 may be required to within a threshold difference of their respectivereference P-wave peak amplitude values. The requirement of P1 being lessthan P2 may be used in some examples, depending on the sensing electrodevectors being used.

FIG. 7 is a timing diagram 300 depicting an example of PWOS detectionand pacing control that may be performed by ICD 14. The first cardiacelectrical signal 302 is a filtered and rectified signal from the firstsensing channel 83. The second cardiac electrical signal 330 is afiltered and rectified signal from the second sensing channel 85. Thefirst sensing channel 83 is configured to control an R-wave sensingthreshold 310 and produce R-wave sensed event signals, e.g., signals340, 342, 344 and 346 in response to the cardiac electrical signal 302crossing the sensing threshold 310.

In this example, the sensing threshold 310 is shown as a multi-levelsensing threshold that is set to a starting value after a post-senseblanking interval 312. The starting value may be based on the peakamplitude of the cardiac signal 302 detected during the post-senseblanking interval 312. The sensing threshold 310 may be adjusted afterpredetermined time intervals to an intermediate value and finally to aprogrammed sensitivity or sensing floor. As described above, however,R-wave sensing threshold 310 may be an auto-adjusted thresholdcontrolled according to a variety of control parameters and techniques.Practice of the PWOS detection techniques disclosed herein are notlimited to a particular method for controlling or setting the R-wavesensing threshold 310.

In this example, a P-wave 304 of the first cardiac electrical signal 302may cross sensing threshold 310 resulting in a sensed event signal 342being produced by the first sensing channel 83. Control circuit 80determines a cardiac event interval 315 since the most recent precedingcardiac event, which is sensed event signal 340 in this example. Thecardiac event interval 315, which may be determined as the value of thepacing interval timer our counter at the time of receiving sensed eventsignal 342, is compared to the PWOS threshold interval. If this cardiacevent interval 315 is less than the PWOS threshold interval, PWOS is notdetected, and the sensed event signal 342 may be treated as a validR-wave for controlling the pacing timing interval.

The pacing interval timer or counter is restarted by control circuit 80in response to the sensed event signal 342, e.g., as shown by interval316. Even though the sensed event signal is actually associated with anoversensed P-wave 304, PWOS is not detected when the cardiac eventinterval 315 is less than the PWOS threshold interval. Since R-wave 306is sensed after P-wave 304, inhibiting a pacing pulse in response tooversensed P-wave 304 does not result in asystole in this example. Bysetting a PWOS threshold interval, incorrect detection of truetachyarrhythmia events as PWOS is avoided. A tachyarrhythmia detectionalgorithm may avoid false detection of tachyarrhythmia due to true PWOSbased on other VT/VF detection criteria designed to detect VT or VF withhigh sensitivity and specificity.

R-wave 306 crosses the R-wave sensing threshold 310 resulting in thenext sensed event signal 344 being produced. Control circuit 80determines the cardiac event interval 316, which is less than the PWOSthreshold interval. The sensed event signal 344 is treated as a validR-wave. The pacing pulse scheduled in response to the preceding sensedevent signal 342 is inhibited, and the pacing interval is restarted bycontrol circuit 80 in response to sensed event signal 344, as indicatedby starting interval 322.

If the P-wave 304 immediately preceding R-wave 306 had not been sensedand sensed event signal 342 not produced, the cardiac event intervalsince the most recent preceding event 340 to sensed event signal 344would be interval 315 plus interval 316, which may be greater than thePWOS threshold interval. This scenario of P-wave 304 not being sensedresulting in a sensed event interval between sensed event signal 340 andsensed event signal 344 may be used to illustrate how P-wave oversensingcriteria avoids falsely identifying an R-wave 306 as PWOS. In thisscenario of P-wave 304 not being sensed such that the sensed eventinterval ending on sensed event signal 344 is greater than the PWOSthreshold interval, control circuit 80 determines the peak amplitudesfrom the first and second cardiac electrical signals 302 and 330 duringpost-sense blanking interval 312. The peak amplitude 314 of R-wave 306would be determined as P1 in this scenario. P1 is compared to thereference P-wave peak amplitude previously established for the firstsensing channel 83. Since the R-waves are generally much larger than theP-waves in the first cardiac signal 302, e.g. 3 to 4 times larger ormore, the difference between peak amplitude 314 and the reference P-waveamplitude is not less than a difference threshold, e.g., 100%, asrequired for detecting PWOS. Since this requirement is not satisfied,PWOS would not be detected based on analysis of the P1 peak duringpost-sense blanking interval 312. Sensed event signal 344 is properlytreated as a sensed R-wave and the pacing interval timer or counter isrestarted as indicated by interval 322.

In some examples, additional PWOS criteria may be applied as describedin conjunction with FIG. 6. If R-wave sensed event signal 344 isreceived at a sensed event interval greater than the PWOS thresholdinterval, the peak amplitude 336 of P-wave 334 of the second cardiacsignal 330 may be determined as P2. The difference between P2 336 andthe reference P-wave amplitude previously established for the secondsensing channel 85 may be compared to a threshold difference. SinceR-wave 334 is much larger than P-waves in the second cardiac signal 330,the difference between P2 336 determined during post-sense blankinginterval 312 and the reference P-wave amplitude determined for thesecond sensing channel is likely to be greater than the thresholddifference, failing to satisfy the PWOS requirement.

The peak amplitudes 314 and 336 may be compared to each other in someexamples. PWOS detection criteria may require that P1 be less than P2.Depending on the sensing electrode vectors being used by the firstsensing channel 83 and the second sensing channel 85 for acquiring theraw cardiac signals corresponding to the rectified, filtered signals 302and 330, R-wave 306 may be expected to be larger than R-wave 336 whereasP-waves 304, 308 in the first signal 302 may be expected to be smallerthan the P-waves 338 of the second signal 330. As such, if peakamplitude 314 is not less than peak amplitude 336, PWOS is not detected.In this way, multiple requirements for detecting PWOS may prevent R-wave306 from being falsely detected as PWOS.

Since the PWOS criteria are not satisfied based on the analysis of peakamplitudes 314 and 336, the pacing timer or counter is appropriatelyrestarted, as indicated by interval 322, in response to the sensed eventsignal 344 corresponding to a valid R-wave 306. The pacing intervaltimer or counter is set according to a programmed pacing rate by controlcircuit 80. For example, the pacing interval timer or counter may be setto expire after 1.0 to 1.5 seconds to provide pacing at a lower rate of60 to 45 pulses per minute, respectively, in the absence of sensedR-waves.

P-wave 308 of first cardiac electrical signal 330 crosses the R-wavesensing threshold 310 causing the next sensed event signal 346 to beproduced to be produced during the pacing interval 322 started inresponse to sensed event signal 344. The post-sense blanking interval313 is started in response to sensed event signal 346. The cardiac eventinterval 320 since the most recent preceding sensed event signal 344 isdetermined, for example by checking how much time of pacing interval 322has elapsed based on a value of the pacing interval timer or counter.Cardiac event interval 320 is compared to the PWOS threshold interval.If cardiac event interval 320 is detected as an interval longer than thePWOS threshold interval, control circuit 80 determines P1 for thecurrent sensed event as the peak amplitude 318 from the first cardiacsignal 302 during post-sense blanking interval 313. Control circuit 80determines P2 for the current sensed event signal 346 as the peakamplitude 340 from the second cardiac signal 330 during the post-senseblanking interval 313.

P1 and P2 may each be compared to their respective reference P-wave peakamplitudes previously established for first sensing channel 83 andsecond sensing channel 85. P1 and P2 may also be compared to each other.If the peak amplitudes 318 and 340 are each within a thresholddifference from their respective reference peak amplitudes and peakamplitude 318 is less than peak amplitude 340, the sensed event signal346 is detected as a PWOS event. Sensed event signal 346 is ignored forthe purposes of inhibiting a pacing pulse and restarting the pacinginterval 322. Restarting of the pacing interval timer or counter iswithheld by control circuit 80 such that the pacing interval 322continues to run.

As described above, other signal features may be determined instead ofor in addition to the peak amplitudes 318 and 340. For instance, theareas of the signal 308 and signal 338 may be determined by summing thesample points during the post-sense blanking interval 313 or a portionthereof. An average signal width may be determined by dividing the areaby the respective peak amplitude 318 or 340. Other signal features sucha peak slope may be determined during the post-sense blanking interval313 from each of cardiac signal 310 and cardiac signal 330 forcomparison to each other and/or to reference values previouslyestablished for the given signal feature.

If R-wave sensing threshold 310 was at the programmed sensitivity orsensing floor when P-wave 308 was sensed, R-wave sensing threshold 310may remain at the programmed sensitivity of sensing floor in response tosensed event signal 346 being identified as a PWOS event. If R-wavesensing threshold 310 was not yet at the sensing floor, sensing circuit86 may continue to adjust R-wave sensing threshold 310 according toauto-adjusting threshold control parameters, which may be based on peakamplitude 314 of sensed R-wave 306. The peak amplitude 318 determinedduring post-sense blanking interval 313 may not be used to adjust theR-wave sensing threshold 310 since sensed event signal 346 is identifiedas a PWOS event. Pacing interval 322 expires without the occurrence of asensed event not detected as PWOS. Control circuit 80 controls therapydelivery module 84 to deliver a pacing pulse 324 in response to thepacing interval 322 expiring.

Thus, an IMD system and method for detecting PWOS on a sensedevent-by-event basis have been presented in the foregoing descriptionwith reference to specific embodiments. In other examples, variousmethods described herein may include steps performed in a differentorder or different combination than the illustrative examples shown anddescribed herein. Furthermore, P-wave oversensing may successfully bedetected in variations that omit one or more steps presented herein. Forexample, the IMD may neglect the oversensed P-waves in determining aventricular rate for detecting ventricular tachyarrhythmias, but notprovide any pacing therapy that is responsive to the P-wave oversensing.This may be particularly the case in an extra-cardiovascular system inwhich leads are be positioned extra-thoracically (outside the ribcageand sternum). It is appreciated that various modifications to thereferenced embodiments may be made without departing from the scope ofthe disclosure and the following claims.

1. A medical device comprising: a sensing circuit configured to: receiveat least one cardiac signal; set an R-wave sensing threshold to astarting value; adjust the R-wave sensing threshold from the startingvalue according to auto-adjusting threshold control parameters; sense acardiac event in response to the at least one cardiac signal crossingthe R-wave sensing threshold; and a control circuit coupled to thesensing circuit and configured to, in response to the cardiac eventsensed by the sensing circuit, determine that P-wave oversensingcriteria are met based on the at least one cardiac signal; wherein thesensing circuit is further configured to continue adjusting the R-wavesensing threshold according to the auto-adjusting threshold controlparameters in response to the control circuit determining that theP-wave oversensing criteria are met.
 2. The medical device of claim 1,wherein the control circuit is further configured to: start a pacinginterval; and withhold restarting of the pacing interval in response todetermining that the P-wave oversensing criteria are met.
 3. The medicaldevice of claim 2, wherein the control circuit is further configured todetect an expiration of the pacing interval; and further comprising atherapy delivery circuit configured to generate a cardiac pacing pulsein response to the expiration of the pacing interval.
 4. The medicaldevice of claim 1, wherein the control circuit is further configured todetermine that the P-wave oversensing criteria are met by: detecting anevent interval from the at least one cardiac signal that is greater thana P-wave oversensing threshold interval, the event interval extendingfrom the cardiac event sensed from the at least one cardiac signal bythe sensing circuit to a most recent preceding cardiac event;determining a first feature from the at least one cardiac signal sensedby the sensing circuit in response to detecting the event interval thatis greater than the P-wave oversensing threshold interval; anddetermining that the P-wave oversensing criteria are met by the firstfeature.
 5. The medical device of claim 4, wherein the control circuitis further configured to: start a pacing interval; set the P-waveoversensing threshold interval based on the pacing interval; andwithhold restarting of the pacing interval in response to determiningthat the P-wave oversensing criteria are met.
 6. The medical device ofclaim 4, wherein: the sensing circuit is further configured to receive asecond cardiac signal; and the control circuit is further configured to:determine a second feature of the second cardiac signal in response todetecting the event interval greater than the P-wave oversensingthreshold; and determine that the P-wave oversensing criteria are metbased on the first feature and the second feature.
 7. The medical deviceof claim 6, wherein the control circuit is further configured todetermine that the P-wave oversensing criteria are met based on thefirst feature and the second feature by: determining the first featureas a first amplitude of the at least one cardiac signal; determine thesecond feature as a second amplitude of the second cardiac signal; anddetermine that the P-wave oversensing criteria are met based on adifference between the first amplitude and the second amplitude.
 8. Themedical device of claim 1, wherein the control circuit is furtherconfigured to determine that P-wave oversensing criteria are met basedon the at least one cardiac signal by determining from the at least onecardiac signal at least one an amplitude, a slope, a signal area, or asignal width.
 9. The medical device of claim 1, wherein the sensingcircuit is further configured to: adjust the R-wave sensing thresholdfrom the starting value to a sensing floor according to theauto-adjusting threshold control parameters; sense the cardiac event inresponse to the at least one cardiac signal crossing the sensing floor;and continue adjusting the R-wave sensing threshold according to theauto-adjusting threshold control parameters in response to the controlcircuit determining that the P-wave oversensing criteria are met byholding the R-wave sensing threshold at the sensing floor.
 10. Themedical device of claim 1, wherein the control circuit is furtherconfigured to determine that the P-wave oversensing criteria are met by:establishing a reference amplitude from the at least one cardiac signal;and determining that an amplitude of the at least one cardiac signalduring a time interval following the sensed cardiac event is within athreshold difference of the reference amplitude.
 11. A non-transitorycomputer readable medium storing instructions which, when executed bycontrol circuitry of a medical device, cause the medical device to:receive at least one cardiac signal; set an R-wave sensing threshold toa starting value; adjust the R-wave sensing threshold from the startingvalue according to auto-adjusting threshold control parameters; sense acardiac event in response to the at least one cardiac signal crossingthe R-wave sensing threshold; and in response to the sensed cardiacevent, determine that P-wave oversensing criteria are met based on theat least one cardiac signal; and continue adjusting the R-wave sensingthreshold according to the auto-adjusting threshold control parametersin response to determining that the P-wave oversensing criteria are met.12. The non-transitory computer readable medium of claim 11 furthercomprising instructions that cause the medical device to: start a pacinginterval; and withhold restarting of the pacing interval in response todetermining that the P-wave oversensing criteria are met.
 13. Thenon-transitory computer readable medium of claim 12 further comprisinginstructions that cause the medical device to: detect an expiration ofthe pacing interval; and generate a cardiac pacing pulse in response tothe expiration of the pacing interval.
 14. The non-transitory computerreadable medium of claim 11 further comprising instructions that causethe medical device to determine that the P-wave oversensing criteria aremet by: detecting an event interval from the at least one cardiac signalthat is greater than a P-wave oversensing threshold interval, the eventinterval extending from the sensed cardiac event to a most recentpreceding cardiac event; determining a first feature from the at leastone cardiac signal in response to detecting the event interval that isgreater than the P-wave oversensing threshold interval; and determiningthat the P-wave oversensing criteria are met by the first feature. 15.The non-transitory computer readable medium of claim 14 furthercomprising instructions that cause the medical device to: start a pacinginterval; set the P-wave oversensing threshold interval based on thepacing interval; and withhold restarting of the pacing interval inresponse to determining that the P-wave oversensing criteria are met.16. The non-transitory computer readable medium of claim 14 furthercomprising instructions that cause the medical device to: receive asecond cardiac signal; determine a second feature of the second cardiacsignal in response to detecting the event interval greater than theP-wave oversensing threshold; and determine that the P-wave oversensingcriteria are met based on the first feature and the second feature. 17.The non-transitory computer readable medium of claim 16 furthercomprising instructions that cause the medical device to determine thatthe P-wave oversensing criteria are met based on the first feature andthe second feature by: determining the first feature as a firstamplitude of the at least one cardiac signal; determine the secondfeature as a second amplitude of the second cardiac signal; anddetermine that the P-wave oversensing criteria are met based on adifference between the first amplitude and the second amplitude.
 18. Thenon-transitory computer readable medium of claim 11 further comprisinginstructions that cause the medical device to determine that P-waveoversensing criteria are met based on the at least one cardiac signal bydetermining from the at least one cardiac signal at least one of anamplitude, a slope, a signal area, or a signal width.
 19. Thenon-transitory computer readable medium of claim 14 further comprisinginstructions that cause the medical device to: adjust the R-wave sensingthreshold from the starting value to a sensing floor according to theauto-adjusting threshold control parameters; sense the cardiac event inresponse to the at least one cardiac signal crossing the sensing floor;and continue adjusting the R-wave sensing threshold according to theauto-adjusting threshold control parameters in response to the controlcircuit determining that the P-wave oversensing criteria are met byholding the R-wave sensing threshold at the sensing floor.
 20. A methodcomprising: receiving at least one cardiac signal; setting an R-wavesensing threshold to a starting value; adjusting the R-wave sensingthreshold from the starting value according to auto-adjusting thresholdcontrol parameters; sensing a cardiac event in response to the at leastone cardiac signal crossing the R-wave sensing threshold; in response tothe sensed cardiac event, determining that P-wave oversensing criteriaare met based on the at least one cardiac signal; and continuingadjusting the R-wave sensing threshold according to the auto-adjustingthreshold control parameters in response to determining that the P-waveoversensing criteria are met.